(Industrially Applicable Field)
The present invention relates to a method whereby combustible matter is gasified in a fluidized-bed furnace, and the resulting combustible gas and fine particles are burned at high temperature in a melt combustion furnace, and the resulting ash is melted therein.
In recent years, it has been demanded to reduce the volume of waste, e.g. municipal refuse, waste plastics, etc., which are generated in large amounts, by incineration, and to effectively use heat recovered from the incineration. Since ash resulting from incineration of waste matter generally contains harmful heavy metals, it is necessary to take some measures, e.g. solidification of the heavy metal components, to dispose of the burned ash by reclaiming. To cope with these problems, JP-B2-62-35004 (Japanese Patent Application Post-Examination Publication, KOKOKU) proposes a method of and apparatus for burning solid matter. In the proposed combustion method, a solid material is thermally decomposed in a fluidized-bed pyrolysis furnace, and pyrolysis products, that is, a combustible gas and particles, are introduced into a cyclone combustion furnace, in which the combustible component is burned at high intensity by pressurized air, and the resultant ash is caused to collide with a wall surface thereof by swirl and thus are melted. This molten material flows down on the wall surface, and the resulting molten slag drops from a discharge opening into a water chamber where it is solidified.
The method disclosed in JP-B2-62-35004 suffers, however, from the disadvantage that, since the whole fluidized bed is in an actively fluidized state, a large amount of unreacted combustible component is carried to the outside of the furnace with the combustible gas produced in the furnace. Therefore, high gasification efficiency cannot be obtained. Further, gasification materials usable in fluidized-bed furnaces have heretofore been small coal having a particle diameter in the range of from 0.5 mm to 3 mm, and finely-crushed waste matter of several millimeters in size. Gasification material that is larger in size than the above will obstruct fluidization; while gasification material that is smaller in size than the above will be carried to the outside of the furnace with the combustible gas as an unreacted combustible component without being completely gasified. Accordingly, the conventional fluidized-bed furnaces necessitate previously crushing a gasification material and making the resulting particles uniform in size by using a crusher or the like as a pretreatment which is carried out before the gasification material is cast into the furnace. Thus, gasification materials which do not fall within a predetermined particle diameter range cannot be used, and the yield must be sacrificed to some extent.
To solve the above-described problem, JP-A-2-147692 (Japanese Patent Application Public Disclosure, KOKAI) proposes a fluidized-bed gasification method and fluidized-bed gasification furnace. In the fluidized-bed gasification method disclosed in this publication, the furnace has a rectangular horizontal cross-sectional configuration, and the mass velocity of a fluidizing gas jetted upwardly into the furnace from the central portion of the furnace bottom is set lower than the mass velocity of a fluidizing gas supplied from two side edge portions of the furnace bottom. The upward stream of the fluidizing gas is turned over to the central portion of the furnace at a position above each side edge portion of the furnace bottom. Thus, a moving bed or descending fluidized bed in which a fluidized medium settles is formed in the central portion of the furnace, and an ascending fluidized bed in which the fluidized medium is actively fluidized is formed in each side edge portion of the furnace. Combustible matter is supplied to the moving bed. The fluidizing gas is either a mixture of air and steam, or a mixture of oxygen and steam, and the fluidized medium is siliceous sand.
However, the method of JP-A-2-147692 has the following disadvantages:
(1) Gasification endothermic reaction and combustion reaction simultaneously take place in all the moving and fluidized beds. Accordingly, a volatile component, which is readily gasified, is burned at the same time as it is gasified, whereas, fixed carbon (char) and tar, which are difficult to gasify, are carried, as unreacted matter, to the outside of the furnace with the combustible gas produced in the furnace. Thus, high gasification efficiency cannot be obtained.
(2) In a case where the combustible gas produced in the furnace is burned for use in a steam and gas turbine combined-cycle power generation plant, the fluidized-bed furnace must be of the pressurized type. In this case, however, since the furnace has a rectangular horizontal cross-sectional configuration, it is difficult to construct the furnace in the form of a pressurized furnace. Preferable gasification furnace pressure is determined by the application of the combustible gas produced. In a case where the gas is used as an ordinary gas for combustion, the furnace pressure may be of the order of several thousands of mmAq. However, in a case where the combustible gas produced is used as a fuel for a gas turbine, the furnace pressure must be as high as several kgf/cm.sup.2. When the gas is used as a fuel for high-efficiency gasification combined-cycle power generation, a furnace pressure higher than ten-odd kgf/cm.sup.2 is suitably used.
In treatment of wastes such as municipal refuse, volumetric reduction by burning combustible refuse still plays an important role. In relation to incineration, there has recently been an increasing demand for environmental protection-type refuse treatment techniques, e.g. dioxin-control measures, techniques for making smoke and dust harmless, improvement in energy recovery efficiency, etc. The rate of incineration of municipal refuse in Japan is about 100,000 tons/day, and energy recovered from the entire amount of municipal refuse is equivalent to about 4% of the electric energy consumed in Japan. At present, the municipal refuse energy utilization factor is as low as about 10%. However, if the energy utilization factor can be increased, the rate of consumption of fossil fuel decreases correspondingly, so that it is possible to contribute to the prevention of global warming.
However, the existing incineration system involves the following problems:
1 The power generation efficiency cannot be increased because of the problem of corrosion by HCl.
2 Environmental pollution prevention equipment for controlling HCl, NO.sub.x, SO.sub.x, mercury, dioxins, etc. has become complicated, resulting in increases in cost and space.
3 There is an increasing tendency to install burned ash melting equipment on account of tightening of regulations, difficulty in ensuring a site for final disposal, and so forth. For this purpose, however, additional equipment must be constructed, and a great deal of electric power is consumed.
4 Costly equipment is needed to remove dioxins.
5 It is difficult to recover valuable metals.